Keplerian type refracting telescope. At the far left is the
convex objective lens. It has a focal length f1 , the length
which it brings light from a distant object to a focus. The convex
secondary lens (focal length f2) redirects the light towards
observer’s eye. The lens in the observer’s own eye then focuses
it on the observer’s retina. In effect the observer is looking at
an image that lies between the two lenses. This image is upside
down ("inverted") compared with the original distant object.
Adapted from an illustration
in The History of the Telescope by H.C. King, Sky Publishing
1955, drawn by Linda Wooliever.